1. Field of the Invention
The present invention relates to an improvement in an electrochemical cell designed by enhancing the manufacture of a housing of the cell, particularly the closure of a fill port. The fill port is used to introduce an electrolyte into the housing of the electrochemical cell.
2. Prior Art
In prior and current product lines, the design of electrochemical cells includes a lid with separate ferrules or openings for providing a terminal pin feedthrough, and an electrolyte fill and closure mechanism. Designs for these functions usually consist of a lid, GTMS ferrule, and an electrolyte fill ferrule which are welded together to form a subassembly. This subassembly is further manufactured by having the GTMS inserted into its ferrule and the completed assembly is then welded into a battery case of various configurations. The battery is filled with electrolyte via the fill ferrule or area, and the fill mechanism comprising a single fill port covering is welded shut. U.S. Pat. No. 5,306,581 to Taylor et al. discloses one such example of an electrochemical cell comprising a single electrolyte fill port covering.
However, these electrochemical cells comprising single electrolyte fill covers are not desirable because these cell designs are typically prone to electrolyte leakage during manufacture. The welding operation of sealing the single cover of the fill port typically causes the electrolyte material near the fill port opening to heat and expel out of the casing of the cell before the welding process is complete and the lid is hermetically sealed. Expelled electrolyte material is not desirable because it generally interacts with the welded material causing discoloration of the weld and may potentially compromise the robustness of the hermetic seal of the electrolyte fill port over time.
Dual weld plug electrochemical cell designs such as the one disclosed in U.S. Pat. No. 6,610,443 to Paulot et al., incorporated herein by reference, were created to help alleviate this problem. In the '443 patent, Paulot discloses an electrochemical cell comprising a header assembly containing both a glass to metal seal opening for a terminal lead and an electrolyte fill opening comprising two inline openings. As shown in FIG. 8C, the Paulot fill port comprises a through bore having an upper and lower opening of differing diameters. A sealing element is positioned over the lower opening and mechanically pressed into an interference fit within the opening. After the lower sealing element is mechanically pressed into position, a second sealing element is positioned within the upper opening and welded shut.
However, the applicants of the present invention have discovered that merely pressing such a sealing element within the fill port opening, as disclosed by Paulot, is not sufficient in preventing potential leakage of electrolyte from within the cell during manufacture. Prior art double plug fill enclosure designs, such as Paulot's, depend on an interference fit of the lower plug within the fill hole to stop the potential leakage of electrolyte from within the electrochemical cell. Variations in plug dimensions and surface conditions increase the difficulty in precisely controlling the insertion force of the plug to minimize electrolyte leakage. In addition, heat from the welding of the sealing element to the case during manufacturing of the cell exacerbates these variations causing a potential break in the temporary seal, which then allows for the electrolyte to leak out from the case.
In addition, during welding of the upper sealing member, heat from the welding process typically expands the components of the cell, thereby possibly additionally compromising the interference fit of the lower sealing member. As a result, additional electrolyte material may leak out from the casing which may then interact with the welded material of the upper sealing member.
If any electrolyte material were to leak out, a cleaning process by which the expelled electrolyte material is removed from within the fill port opening is required before the fill port opening is hermetically sealed. Removal of electrolyte from within the fill port opening is not desirable. The relatively small diameter of the fill port opening increases the difficulty of electrolyte removal. The electrolyte removal process increases cost and time of manufacture of the electrochemical cell.
International patent publication number WO 92/10859 to Cretzmeyer et al. discloses an electrochemical cell also having a dual opening fill port design. As disclosed, an inner plug composed of a compressible material is positioned and mechanically pressed within an inner fill port opening. A helium holding material, such as glass beads, is positioned over the inner plug and a metal closure button is welded over the fill port opening, sealing the electrolyte within. However, like Paulot, the closure of the Cretzmeyer electrolyte fill port relies on a plug that is mechanically pressed within the lower fill port opening. Such a mechanically pressed plug does not adequately stop leakage of electrolyte material during welding of the fill port closure.
Thus, as previously stated, the problem with the prior art lid or header designs is that during the electrolyte fill port enclosure procedure, electrolyte material typically escapes out of the fill port thus resulting in a loss of electrolyte material. Furthermore, during welding of the fill port opening, heat from the welding causes localized heating of the metallic casing and electrolyte materials. This localized heating can cause the electrolyte to expel and leak out from within the casing. Such expulsion of the electrolyte material typically results in discoloration of the welded fill enclosure metal as the electrolyte material interacts with the welded metal. In addition, such an interaction of the electrolyte material with the fill opening enclosure material, during welding, may result in a change in the microstructure of the welded seal. Such a change in the weld microstructure may compromise the robustness of the hermetic seal over time.
The new design, the subject of the current patent application, thus reduces expulsion of electrolyte during cell assembly by utilizing a fill port opening comprising multiple welded seals. The new design allows for the creation of a hermetic electrolyte fill port opening with minimized or eliminated expulsion of the electrolyte material.